1. Field of the Invention
The present invention relates to a fiber glass composition and, in particular, to a glass composition for glass fibers adaptable for use in reinforcement of printed wiring boards.
2. Description of the Prior Art
Among various types of the glass fiber, a so called "E" glass fiber has been used in the electrical or electronic technical field, for example, for printed wiring boards, because of its unique electrical insulation properties, as disclosed by G. N. Bolen in the MANUFACTURE OF GLASS-FIBER YARNS FOR PRINTED WIRING BOARDS, 29th National SAMPE Symposium, Apr. 3-5, 1984.
A kind of E fiber glass is disclosed in U.S. Pat. No. 2,334,961 by Schoenlaub, which consists, by weight, of SiO.sub.2 52-56%, Al.sub.2 O.sub.3 12-16%, CaO 16-19%, MgO 3-6%, and B.sub.2 O.sub.3 9-11%.
U.S. Pat. No. 2,571,074 by Tiede et al discloses another kind of E glass which consists, by weight, of SiO.sub.2 52-56%, Al.sub.2 O.sub.3 12-16%, CaO 19-25%, and B.sub.2 O.sub.3 8-13%.
However, a printed wiring board using E glass fiber is not proper for use in high speed digital signal processing devices because it has a high dielectric constant (about 5.5) which causes unacceptable signal propagation time.
As another type of glass fiber having an improved dielectric constant, a so called "D" glass fiber is disclosed in the "Handbook of Fillers and Reinforcements for Plastics", published by VAN NOSTRAND REINHOLD COMPANY, p.p. 480 and 481, and consists, by weight, of SiO.sub.2 74.5%, Al.sub.2 O.sub.3 0.3%, B.sub.2 O.sub.3 22.0%, CaO 0.5%, Li.sub.2 O 0.5%, Na.sub.2 O 1.0%, and K.sub.2 O 1.5%. The D glass fiber has a dielectric constant of 3.56 for 10.sup.6 Hz, at 72.degree. F.
Although the D glass fiber has a desirable dielectric constant, it has a problem in that it is not water proof or water resistant. That is, the D glass has a low resistance to water and the alkaline elements are eluted into the water. Accordingly, the D glass fiber exhibits low adhesion to plastic resin so that the reliability of a printed wiring board using the D glass is degraded.
Further, the D glass fiber has a low strain point of about 470.degree. C. and is, therefore, has a low resistance to heat. The glass fiber is heat-treated at 380.degree. C. for 72 hours to fire the binder adhered to the fiber prior to application into the printed wiring board. At that time, the D glass fiber is apt to deteriorate because of its low strain point.